The original launch of the game was about 25GB.
I used to be able to run it off a 32GB thumb drive until mid 2015 or so when it went past 32GB. It's now about 37 GB like the above have said... if you see the size is less, it's probably because it hasn't fully patched yet.
so nowdays you can run it off a 64GB thumb or SD drive if ye like.
(running off a thumb drive allows you to run it on different PCs. Work, friends, home desktop, laptop, cousin's...